Underfloor void

Low-rise housing Suspended floor slab Ventilated underfloor void... 

For low-rise housing with gardens, where a ventilated underfloor void is provided, the CSV is used in Table 6.4 (see colour section) to determine the level of risk (traffic light classification green to red). 

In this case the results of the risk assessment showed that for the houses located more than 50 m from the edge of the gas source there was negligible risk (less than 1 x 10 ) from ground gas migration, even when using the highest gas concentration ever recorded on the site. Therefore it was considered that gas protection (over and above a ventilated underfloor void) was unlikely to be necessary for the houses that were more than 50 m from the source. At a distances between 5 m and 50 m from the source the risk... 

It should also be ensured that the construction of extensions does not in any way compromise the effectiveness of existing gas protection measures (e.g. by blocking vents to the underfloor void or damaging existing membranes). 

The risk assessment can be extended using the gas monitoring data and the site characterisation system from ClRlA Report C 659/665 or the NHBC traffic light system (for low-rise housing with an underfloor void). 

Ventilation below buildings (usually an underfloor void or venting layer). This can be active or passive... 

Underfloor voids can take a number of different forms ... 

Active venting uses fans to enhance the airflow through underfloor voids. The number of fans provided depends on the required air flow. It is best if fans that allow passive air flow when they are not operating are used and that a gas monitoring system is installed in the underfloor void. The monitoring system activates the fans to increase air flow when the gas levels exceed set values. 

The best place to use gas monitoring is in an underfloor void so that advance warning of gas migration is given before it builds up in occupied spaces. 

Underfloor void, high-level alarm 40% LEL of methane... 

Amber 1 Low-level gas protection using a membrane and ventilated underfloor void in accordance with BRE Report 414, ventilated void to give at least one air change per day. 

I Amber 2 1 High-level gas protection using a membrane and ventilated underfloor void in accordance with BRE Report 414, ventilated void to give at least one air change per day. Membrane to be installed by specialist contractor and the membrane and ventilation to be certified as being installed correctly. This requires that the membrane is installed under a CQA regime by a specialist and is tested after installation (Appendix E3 of Boyle and Witherington, 2007). In effect this means that specialist gas-resistant membranes will be required. 

The traffic light classification is provided to give an indicative comparison only as the actual boundaries between the levels are different (see Box 6.3). The traffic light classification is restricted to low-rise housing with a ventilated underfloor void. 

In both cases the effect of differential femperatures between the outside air and the underfloor void are ignored. In practice this can induce significant air flow. 

It is possible to design passive systems so that the pipework and electrical connections are in place to allow an upgrade to an active system if that is required. The underfloor void of the passive system is monitored. If monitoring indicates that the passive system is not working adequately, active fans can be easily and quickly installed. This has been done on projects where the desk study has identified a... 

The scope of a gas protection system is based on the results of the risk assessment. A combination of measures is usually used to minimise the risks associated with ground gas. This is so that if one element of the protection fails the others will continue to protect the development. Once the scope has been defined, detailed design of individual elements such as the venting or membranes can be completed. The latest QRIA guidance has two methods of defining the scope of gas protection one that is for low-rise housing and a ventilated underfloor void (minimum 150 mm high) the other is for any other type of development. 

Internal monitoring should be considered a last resort, not a means of protection. The best place for monitoring points is in the underfloor void or vent system. 

Post-construction monitoring of underfloor voids is usually resisted by housing developers and is not widely undertaken. However, it is possible to monitor die imderfloor voids between membrane installation and selling of a property and this has been carried out on a number of sites. Post-construction monitoring has also been used to demonstrate the effectiveness of underfloor voids in reducing the risk of gas ingress where gas protection membranes have been omitted by mistake or where validation has not been imdertaken. 

---